Emerging concepts in male contraception: a narrative review of novel, hormonal and non-hormonal options

Access to reliable contraception is a pillar of modern society. The burden of unintended pregnancy has fallen disproportionately on the mother throughout human history; however, recent legal developments surrounding abortion have sparked a renewed interest in male factor contraceptives beyond surgical sterilization and condoms. Modern efforts to develop reversible male birth control date back nearly a century and initially focused on altering the hypothalamic-pituitary-testes axis. These hormonal contraceptives faced multiple barriers, including systemic side effects, challenging dosing regimens, unfavorable routes of delivery, and the public stigma surrounding steroid use. Novel hormonal agents are seeking to overcome these barriers by limiting the side effects and simplifying use. Non-hormonal contraceptives are agents that target various stages of spermatogenesis; such as inhibitors of retinoic acid, Sertoli cell–germ cell interactions, sperm ion channels, and other small molecular targets. The identification of reproductive tract–specific genes associated with male infertility has led to more targeted drug development, made possible by advances in CRISPR and proteolysis targeting chimeras (PROTACs). Despite multiple human trials, no male birth control agents have garnered regulatory approval in the United States or abroad. This narrative review examines current and emerging male contraceptives, including hormonal and non-hormonal agents.


Introduction
Reliable family planning is an essential element of modern society, with unplanned pregnancies inflicting a significant financial and emotional burden on both the individual and societal levels. For much of human history, the socioeconomic burden of unintended pregnancies has fallen disproportionately on the mother, with its associated financial consequences. The US News and World Report predicts an average cost of $267,000 to raise a child to 18 years of age. 1 There is also a significant economic burden on the United States taxpayer from unplanned pregnancies, with estimates between $5.5 billion and $21 billion per year. [2][3][4] Over the last century, there has been significant progress to ensure safe, reliable, and cost-effective female contraceptives, such as the birth control pill, intrauterine devices (IUDs), emergency contraceptives, diaphragms, vaginal rings, subcutaneous injections, intramuscular (IM) injections, and implants. 5,6 Despite these efforts, approximately half of all pregnancies are unplanned. 7 Recent estimates suggest that there are 121 million unintended pregnancies annually worldwide, with 61% of these pregnancies ending in abortion. 8 Recent data in the United States suggests that 45% of pregnancies are unplanned, and 40% of these pregnancies end in abortion. 9 With recent rulings in the United States Supreme Court journals.sagepub.com/home/reh Therapeutic Advances in Reproductive Health 17 casting doubt on access to legal abortion, there has been a renewed need for reliable family planning methods. 10 The ability of men to actively participate in family planning with male centered contraceptives has been limited by the number of options, primarily condoms or vasectomy. Estimated numbers from the United Nations Department of Economic and Social Affairs report in 2019 showed that male centered birth control accounted for only 28% of worldwide contraceptive use (male condoms 21%, vasectomy 2%, and withdrawal 5%). 6 These values are only slightly higher when looking at North America (total 30.9%, male condom 17.6%, vasectomy 6.9%, and withdrawal 6.4%) and Europe (total 39.7%, male condom 29.2%, vasectomy 3%, withdrawal 7.5%). 6 Over the last half century, there has been an increased interest in developing reliable and safe male birth control options other than the condom or vasectomy. Between 25% and 71% of men report that they would use a male birth control option analogous to the female pill if one was commercially available. 11,12 Throughout the last several decades, there have been multiple hormonal and non-hormonal birth control options with promising results; however, no option has garnered the US Food and Drug Administration (FDA) approval. In this review, we examine the past and current agents of male birth control research.

Condoms
The origin of male birth control is debated by scholars, but likely dates back to condom use in ancient Egypt and Greece. The first recorded use of a condom is found in the legend of Minos and Pasiphae, written by Antoninus Liberalis in the second century AD. Pasiphae is said to have used a goat's bladder condom to protect her from evil serpents and scorpions in her unfaithful husband's semen. 13 In the 10th century AD, the Persian physician Al-Akhawayni encouraged the use of animal gallbladder condoms. These 'skin' condoms were eventually replaced by rubber in 1844 and latex in the 1930s.
Male condom use is associated with a 2-3% unintended pregnancy rate with perfect use and 12% with typical use. 5,14 There have been many public service campaigns to encourage consistent and proper condom use for the prevention of sexually transmitted infections (STIs) and unintended pregnancies. One national study of 5865 US adolescents and adults aged  showed that only 21.5% of men had used a condom at least once in the last 10 vaginal intercourse encounters. 15 Another cross-sectional national survey of adults aged  showed an overall prevalence of condom use of 24.8% at their last sexual encounter. 16 In addition, the use of condoms has been shown to be associated with mistrust in relationships or to interfere with intimacy. 17,18 There are also condoms developed with spermicidal agents, but poor evidence to suggest any greater benefit in preventing unwanted pregnancies. 19

Vasectomy
The first documented vasectomy was performed in 1823 by Sir Ashley Cooper on a dog. 20 Gosselin continued this research with human cadaver studies and further experimenting with vasectomy techniques on dogs in 1847. The first human vasectomy is credited to R. Harrison in London. 20 Vasectomy was not initially seen for its value as a contraceptive. Multiple surgeons of the day employed vasectomy with the intent of prostatic atrophy. The Austrian physician Eugen Steinach also purported the use of unilateral vasectomy to restore vigor in older gentleman, a concept known as rejuvenescence. 21 The history of vasectomy took a darker turn when it was identified as a means to forward the eugenics movement. In 1897, A. J. Ochsner performed the first vasectomy in the United States, which he saw as a means of sterilizing criminals and slowing 'racial degeneration'. 22 This disturbing work continued in 1902, when Harry C. Sharp sterilized 42 inmates at the Indiana Reformatory to prevent the birth of future criminals. 23 It was around the time of the Second World War when the vasectomy was recognized as a viable form of consensual male sterilization. 20 Approximately 500,000 vasectomies are performed annually in the United States, with 5-10% of married men having undergone the procedure. 5,24 The unintended pregnancy rate in the first year following vasectomy is between 0.02% and 0.1%. 5,25 This is a relatively quick and reliable method of sterilization with low complication rates of 1-2%. 26 One of the major barriers for widespread use of vasectomy is the permanence journals.sagepub.com/home/reh 3 of the procedure, with the associated high cost and uncertainty of vasectomy reversal success. The complication rate of vasectomies is most determined by the procedure volume, with one study reporting that doctors performing more than 50 procedures per year had one third the complication of their counterparts performing less than 10. 27 The most common complications are hematoma (2%), infection (3-4%), sperm granuloma (40%), and persistent post-vasectomy pain (1-14%). 28 The reversibility of the procedure is an important consideration for many patients with the success of the procedure being related to the method of vasectomy and the duration of obstruction. For men who underwent a reversal less than 3 years after their initial operation, patency was reported as >95% and a pregnancy rate was reported at 75% with both rates decreasing as duration of obstruction increased. 29

Hormonal regulation of spermatogenesis
Normal semen parameters have been established by international studies sponsored by the World Health Organization (WHO), in which 3589 semen samples were analyzed. The selected men were all deemed fertile with a partner's time to pregnancy of less than 12 months. They defined a normal sperm concentration as greater than 15 million/ml of ejaculate. 30 With normal semen parameters, 75% of couples will achieve pregnancy within 6 months, and 85% of couples will conceive in 1 year. 31,32 The perfect male contraceptive would lead to azoospermia, where there are no motile sperm identified on semen analysis. This ideal is often unachievable, and a more realistic real-world goal would be a sperm concentration of less than 1 million/ml. The target of <1 million/ml has been shown to result in a pregnancy rate of less than 1% per year, which is similar to female hormonal contraceptive pills. 33 This target sperm concentration has been deemed acceptable by the American Society of Andrology. 34 Beyond efficacy, the other main criteria for an acceptable male contraceptive are reversibility and lack of systemic side effects. 35 The first attempts to develop male birth control by lowering sperm levels were targeted at the hormonal axis between the hypothalamus, pituitary gland, and testes. In healthy, eugonadal men, this axis operates with a negative feedback loop to regulate spermatogenesis and steroidogenesis, as shown in Figure 1. Pulsatile release of gonadotropin-releasing hormone (GnRH) from the hypothalamus is transported via a portal vascular system to the anterior pituitary. 36

Advent of hormonal options for male birth control
Mankind has known about the effects of androgen deprivation for centuries. Aristotle wrote about the effects of castration and the subsequent inability to reproduce, which is regarded as the first scientific article on infertility. 38 Arnold Adolph Berthold, the father of modern endocrinology, performed testicular transplantation experiments in 1849 with roosters, where he noted the return of masculine characteristics after the transplantation of testicles into previously castrated roosters. 38,39 In 1935, testosterone was isolated from bull testes by Ernest Lacquer and simultaneously synthesized in the laboratory by Adolf Butenandt and Leopold Ruzicka. 38 The idea of targeting the hypothalamic-pituitary-testes axis for male birth control was formulated after witnessing the effects of testosterone supplementation in healthy men. This was done initially with either injections, oral formulations, or topical applications.

Testosterone enanthate
In 1939, 4 years after the isolation of testosterone, Norris J. Heckel described decreased spermatozoa count in a 67-year-old man with daily subcutaneous injection of 10-25 mg of testosterone enanthate (TE). Sperm counts returned to normal after the cessation of the injections. 40 The first clinical trials looking at sperm count journals.sagepub.com/home/reh Therapeutic Advances in Reproductive Health 17 suppression with testosterone supplementation were conducted in the 1970s at the National Institutes of Health. These studies showed efficacy in causing oligospermia or azoospermia in the majority of participants. [41][42][43] The WHO conducted two landmark studies in the 1980s and 1990s. The first was a multicenter, international prospective trial with 271 healthy, fertile men. They injected 200 mg TE IM weekly and noted 65% became azoospermic in 3 consecutive semen samples with a mean time to azoospermia of 120 days. There was one recorded pregnancy during 1486 months of the efficacy phase. Importantly, the time to recovery (sperm concentration of at least 20 million/ml) was 3.7 months after discontinuation and 6.7 months to baseline semen parameters. 44 The second WHO study included five additional centers, two additional countries, and 399 healthy men. With the same regimen, they noted four pregnancies for 49.5 person-years involving men with oligozoospermia and 0 pregnancies during 230.4 person-years in azoospermic men. 45 These studies demonstrated efficacy and reversibility of TE injections, but also highlighted several key limitations to this formulation. Azoospermia was only induced in approximately 65% of participants, the frequent injection schedule was not desirable, and there were systemic side effects of unopposed testosterone supplementation; acne, weight gain, altered mood, and changes in hematologic and lipid profiles. 35 Testosterone undecanoate Due to the undesirable injection schedule of testosterone enanthate, there were attempts to study oral formulations of testosterone, namely testosterone undecanoate (TU). The first study with oral TU 80 mg three times daily showed that this dose was insufficient to suppress gonadotropins for sustained azoospermia in six out of seven volunteers. 46 TU monotherapy was abandoned due to this sperm 'rebound'.

Testosterone and progesterone combination therapy
With advancement of male endocrinology, attempts were made to improve sperm suppression, time to suppression, and decrease side effects by using combination regimens of testosterone and progesterone. Multiple of the following regimens also introduce alternate routes of delivery, namely topical gels and implantable pellets.

Progestin cyproterone acetate
Progestin cyproterone acetate (CPA) is a 'firstgeneration' progestin with antiandrogen effects used for treatment of androgen driven processes such as advanced prostate cancer, acne, precocious puberty, excess body hair growth, female birth control, and puberty blockers in transgender females. 47 CPA was first tested as a single agent for male contraception in 1980 in 25 men. 48 Doses of 0, 5, or 10 mg/day were given for 16 weeks, then participants were followed for 24 weeks with semen and serum assays. They noted significant decreases in sperm concentration, motility, morphology, serum testosterone, and gonadotropins. The decrease in androgen levels made this an unlikely candidate for single agent use. Later studies looked at testosterone enanthate in combination with high (25 mg/day) and low (12.5 mg/day) dose CPA for male contraception in the 1990s. 49 They studied 10 normal men for 16 weeks and noted azoospermia in all men in the high-dose arm and 60% of the lowdose arm. Importantly, there were no changes in serum testosterone levels, and gonadotropin drops were transient. Side effects include gynecomastia, sexual dysfunction, bone density loss, and depression. CPA is no longer used in the United States and has an unlikely future as a component in male birth control.

Levonorgestrel implants
Levonorgestrel (LNG) is a progestin commonly used in female birth control and emergency contraception. There are multiple routes of delivery for LNG, including oral, skin patch, IUD, and subcutaneous implant. 50 It was studied as an implant for male contraception in combination with TU injections. This study included 62 Chinese men at three dosage groups. Groups I and II received four LNG subcutaneous rods (75 mg each) followed by either 500 mg or 1000 mg of TU IM every 8 weeks, respectively. Group III was a control, with only 1000 mg TU IM every 8 weeks. They found rates of oligospermia (sperm concentration < 3 million/ml) of 95%, 100%, and 86% in the three groups, respectively. Group II trended toward the highest percentage of azoospermia (90%), but this was not significant. There were no significant side effects or changes in baseline serum testosterone or gonadotropins.

Etonogestrel implants
Etonogestrel (ENG) has been used since the 1990s as a long-acting, progestin female contraceptive in the form of subcutaneous implants or vaginal rings. 51 In the early 2000s, ENG was studied as a male contraceptive in a double-blind, multicenter, placebo-controlled study at high and low doses in combination with TU injections. 52 There were 354 healthy men treated for at least 42 weeks and followed for an additional 24 weeks post treatment. They found that azoospermia was achieved in 89-94% of men by 16 weeks and sustained in 91% of men on treatment. Recovery to sperm concentrations > 20 million/ml was 15 weeks. Side effects of this treatment included weight gain, mood changes, acne, sweating, and loss of libido.

Desogestrel
Desogestrel is an oral progestin used in combination with an estrogen agent or individually in female birth control and to treat menopausal symptoms. 53,54 A multicenter, prospective, randomized trial was conducted with 38 Caucasian and 36 Chinese men to look at the use of low (150 µg daily) and high (300 µg daily) dose oral desogestrel and testosterone pellets (400 mg weeks 1 and 12). 55 The high dose group achieved azoospermia at a significantly higher rate than the low dose group (100% versus 71%, p < 0.05). The testosterone concentrations remained normal, but side effects included lower HDL in caucasian participants and more weight gain in both groups. looking at NETE or NETA in combination with TU injections. 56 Groups I and II received lowdose (200 mg) or high-dose (400 mg) NETE every 6 weeks with 1000 mg TU every 6 weeks, respectively. Group III received 10 mg daily NETA with 1000 mg TU every 6 weeks. These regimens produced sperm concentrations < 1 million/ml in all participants with azoospermia in 13/14 (group I), 11/12 (group II), and 12/14 (group III). The side effects included increase in body weight, erythrocytes, hemoglobin, and hematocrit and decrease in HDL and alkaline phosphatase. They also noted an increase in liver enzymes in the oral NETA group.

Medroxyprogesterone acetate
Medroxyprogesterone acetate (MPA) is an oral or injectable progestin used commonly in depot formulation for female birth control. It also treats menopausal symptoms, endometriosis, abnormal uterine bleeding, certain cancers, precocious puberty, and male paraphilia. 47,57 The depot formulation (300 mg IM every 3 months) was studied in tandem with testosterone implants (every 3 or 4 months) in 55 healthy men as a form of longacting male birth control. 58 Once the men reached azoospermia, they entered a 12 month efficacy period. They reported no pregnancies in 426 person-months. There were two men (3.6%) who did not reach azoospermia. Once the efficacy period was terminated, there was complete recovery to sperm concentration > 20 million/ml in 5 months.

Novel hormonal agents
Prior testosterone and progesterone treatment options have been inconvenient for study participants due to the multiple routes of delivery and dosing regimens. Current hormonal options are not ideal for patients due to several factors including adverse side effect profiles of testosterone excess, failure rates of current hormonal agents, and rates of recovery of spermatogenesis. 59 Novel hormonal agents have aimed to simplify the route of delivery and dosing regimen to make them more convenient for patients.

Nestorone and testosterone gels
Segesterone acetate (SGA) is a progestin that has no androgenic, estrogenic, or glucocorticoid activity. Nestorone (NES), the brand name equivalent of SGA, has been formulated as a transdermal gel, making it a possible companion for combined testosterone and progestin gel birth control. One of the first male birth control studies to employ daily NES gel analyzed the effect on serum gonadotropin levels with and without combined testosterone gel in 2009. 60 This was a randomized, unblinded, dual-center study with 140 healthy men. There were seven groups of 20 men who received between 2 and 8 mg of daily NES gel with or without 10 mg testosterone gel. They found the highest levels of gonadotropin suppression (<0.5 IU/L) in the combined groups, suggesting a need for further research into effects on sperm concentration with the combined NES plus testosterone regimen.
The first study to look at the effects of NES gel plus testosterone gel on sperm concentrations was in 2012. 61 This was a randomized, doubleblind, dual-center study with 99 healthy men. There were three treatment groups, each with 10 mg testosterone gel plus 0, 8, or 12 mg NES gel. These were administered as two gels. The primary endpoint was azoospermia at 20-24 weeks. They found similar rates of azoospermia in the testosterone plus 8 mg NES group and testosterone plus 12 mg NES group (89% and 88%, respectively). These were significantly higher than the placebo group (23%, p < 0.0001 and p = 0.0002). The median serum free testosterone and total testosterone were also maintained in the normal range. Only 57% adhered to the protocol, prompting the group to look at patient satisfaction. Serious side effects were minimal, five men discontinued the trial due to irritability, nightmares, decreased libido, increased appetite, mood swings, and asthma exacerbation. The remaining participant discontinuations were due to inconvenience of the dosing regimen and frequency of study related visits. In 2014, the group reported a 56% satisfaction with the regimen, and only 33% said they would use the two gels as their primary form of contraception. 62 In order to improve compliance, a single gel formulation with combined NES and testosterone was developed (NES-T). 63 This daily, single gel birth control was studied in 44 healthy men in a double-blind, controlled trial. 63 They found that 84% of the NES-T group had suppressed serum gonadotropins by day 28 compared to 16.7% in journals.sagepub.com/home/reh 7 the T-only gel group (p < 0.001). Importantly, 80% of the men were satisfied with the regimen and over 50% said they would use the single gel formulation as their primary form of contraception if available on the market.

Dimethandrolone undecanoate
Dimethandrolone undecanoate (DMAU) is a synthetic precursor drug of dimethandrolone (DMA), which acts on both androgen and progesterone receptors in the body. The dual androgen and progesterone activity have made it an appealing target for single agent male birth control. There are oral and IM formulations of this drug. A randomized, double-blind, phase I trial in 2014 looked at safety for doses of DMAU between 25 and 800 mg versus placebo. 64 At doses above 200 mg of oral DMAU, serum gonadotropins and sex hormones were significantly depressed when taken with food. There were no serious adverse events, changes in vital signs, or laboratory changes up to concentrations of 800 mg. In 2019, a double-blind, randomized, placebo-controlled study with 100 healthy men looked at serum testosterone and gonadotropin levels at various doses of oral DMAU for 28 days. 65 They found that doses of 200 mg per day were sufficient to markedly reduce serum testosterone, LH, and FSH. Acceptability surveys showed that 87% of men receiving the active medication were satisfied with this birth control method, 91% reported no difficulty taking the pills with a high-fat meal, and more than half would use it as their primary form of contraception. 66 is a synthetic androgen with activity at both androgen and progesterone receptors. 67 It is more potent than testosterone and has more resistance to 5-α-reductase. 68 MENT acetate was developed as a long-term birth control using ethylene vinyl acetate implants. This drug was studied in the early 2000s in 35 men who received between one and four subdermal implants (each implant released approximately 400 µg per day). These were maintained for 6-12 months. They found an increase in MENT serum levels with suppressed testosterone, LH, and FSH levels. Azoospermia occurred in 67% (8/12) of the men in the fourimplant group. In 2007, MENT acetate implants were studied in combination with ENG implants versus testosterone pellets with ENG implants. 69 They achieved azoospermia in 80% of men in the MENT group within 12 weeks. Unfortunately, suppression was not maintained due to decrease in MENT release from the implants over time.
Side effects included loss of libido in 60% of men. They also noted decreased serum prostate-specific antigen (PSA) with no change in bone mass, which is due to the resistance to 5-α-reduction and lower activity in the prostate.
is a novel androgenic steroid with progestogenic activity, identified for its specificity in suppressing spermatogenesis with a lack of systemic side effects. It is a prodrug of 11β-methyl-19-nortestosterone (11β-MNT), the biologically active compound. 11β-MNTDC does not undergo aromatization and therefore has fewer expected estrogenic effects. It is also more resistant to 5-α-reduction and therefore has fewer expected androgenic side effects than testosterone. This drug was studied in a randomized, double-blinded, placebo-controlled, phase I clinical trial in 2019. 70 Single oral doses (0, 100, 200, 400, and 800 mg) were given with or without food. They found that ingestion with food significantly increased serum 11β-MNTDC and 11β-MNT levels. They found that doses up to 800 mg were safe and doses between 200 and 800 mg were sufficient to significantly suppress serum testosterone levels. Future studies are warranted to examine gonadotropin suppression and its effects on spermatogenesis.

GnRH antagonists
GnRH antagonists act by suppressing LH and testosterone through competitive inhibition of pituitary GnRH receptors. In a randomized clinical trial in 2004, different GnRH antagonist preparations were found to cause azoospermia in 39 of 47 subjects, and more recently, acycline has been tested in contraceptive trails, although its primary use is in the treatment of prostate cancer. 71 Despite promising results, GnRH antagonists require daily or weekly injections, and incur high costs, leading to many drug developers to not pursue them further (Table 1).

Dimethandrolone undecanoate (DMAU)
A synthetic 1-norandrogen that is a prodrug of dimethandrolone (DMA), an agonist of both androgen and progesterone receptors.
Thirumalai et al. 65 100 Daily oral administration of DMAU (at different concentrations) for 28 days Doses 200 mg markedly suppressed serum T, LH, and FSH in subjects.

7-Alpha-methyl-19nortestosterone (MENT)
A synthetic androgen with activity at the androgen and progesterone receptors causes testosterone suppression Walton et al. 69 35 Received between 1 and 4 subdermal implants (each implant released approximately 400 µg per day) The drug led to azoospermia in 80% of men within 12 weeks A novel androgenic steroid with progestogenic activity, identified for its specificity at suppressing spermatogenesis with lack of systemic side effects.

Novel non-hormonal agents
Since the 1950s, researchers have realized that spermatogenesis can be suppressed without directly acting on the hypothalamus-pituitarytestes axis. Multiple non-hormonal agents have been identified that either irreversibly or reversibly disrupt sperm maturation. These agents may be good candidates for male contraception when compared to hormonal agents, avoiding the side effects that occur when altering the hormonal pathways, the stigma of hormone supplementation (anabolic steroid use in sports), and the difficulties of the dosing regimen or route of administration of hormonal agents.
These non-hormonal agents can theoretically act at any stage of spermatogenesis. Common targets include the testicular retinoic acid receptor, Sertoli cell-germ cell interactions, testicular epigenetics, and an assortment of small molecule inhibitors.

WIN 18,446
Retinoic acid plays an essential role in spermatogenesis, assisting with development of the blood-testis barrier (BTB), spermatogonial differentiation, and spermiation. 72 Male mice with knock-outs of the retinoic acid receptor α (RARα) are infertile. 73 In the late 1950s, researchers studying antiparasitics noted that rats receiving retinoic acid inhibitors developed the unintended consequence of infertility. This realization led to human studies of retinoic acid inhibitors as the first potential non-hormonal male contraceptives. 74 During the initial human studies at the Oregon State Penitentiary, approximately 60 inmates remained azoospermic for 1 year while receiving the drug designated WIN 18,446. Unfortunately, when one of the inmates obtained access to contraband whiskey, they became incredibly ill with what was later identified as a disulfiram reaction. This is due to the mechanism of the drug, targeting aldehyde dehydrogenase 1A2 in the testes to block the production of retinoic acid. Off-target action on aldehyde dehydrogenase 2 in the liver causes accumulation of serum acetaldehyde, with the associated systemic symptoms. The use of WIN 18,446 was ultimately abandoned, but recent attempts have been made to develop retinoic acid receptor antagonists more specific to the testes.

BMS-189453
Bristol-Myers-Squibb (BMS) has developed a series of retinoic acid receptor antagonists with varying degrees of specificity to the testes. BMS-189453 is a panretinoic acid receptor antagonist developed in the 1990s with action on the α, β, and γ receptors. 75,76 In rat models, administration of doses between 5 and 240 mg/kg daily of BMS-189453 resulted in marked testicular degeneration and atrophy. 75 Doses higher than 240 mg/kg resulted in severe toxicity and death. Other studies looked at lower doses of BMS-189453. Chung et al. 77 used 5 mg/kg for 2 weeks or 2.5 mg/kg for 4 weeks and found that all mice were sterile at 4 weeks with return of fertility by 20 weeks. Even doses as low as 1 mg/kg for 4 to 16 weeks resulted in 100% sterility with a return of fertility after termination of the drug. 78 Other studies have looked at more α-selective antagonists such as BMS-189532 and BMS-189614. 79,80 Despite promising in vitro studies, when given to mice at 2 and 10 mg/kg orally for 7 days, these medications showed lower potency than non-selective antagonists. 76,80 Normal spermatid formation and sperm release were observed when given orally, but expected defects in spermatogenesis were seen with intravenous formulations. This suggests that factors, such as hepatic metabolism, higher plasma protein binding, or decreased testicular permeability are decreasing the bioavailability of the α-selective agents.

YCT529
YCT529 is an α-selective retinoic acid antagonist recently presented at the American Chemical Society national meeting 2022. 81 When given to mice orally for 4 weeks, it reduced sperm counts and prevented 99% of pregnancy. There were no systemic side effects seen with less specific retinoic acid receptor antagonists. Importantly, mice could reproduce for 4-6 weeks after stopping the medication. Reportedly, human clinical trials will begin this year.

Indenopyridine derivatives
Indenopyridine derivatives, such as CDB-4022 and RTI-4587-073(l), have been shown to inhibit mature sperm production in rat, primate, and 10 journals.sagepub.com/home/reh Therapeutic Advances in Reproductive Health 17 stallion models. [82][83][84] These molecules work primarily by disrupting the attachment of immature spermatids from the seminiferous tubules, causing sloughing of immature, non-motile germ cells into the semen. Hild et al. 82 studied the effects of a single dose of CDB-4022 on rat testicle ultrastructure. They noted degenerative changes in both Sertoli cells and spermatids. The Sertoli cells had an increase in the number of vacuoles, cellular debris, swollen mitochondria, and swollen endoplasmic reticulum. The spermatids had diffuse chromatin and broken nuclear envelopes. Primate studies with the same molecule showed sperm concentrations lower than 1 million/ml by 17 days and remained suppressed for 6 weeks. 83 More interestingly, they found sperm motility had dropped to 0% with immature spermatids present. Serum Inhibin B was elevated, but testosterone, LH, FSH, and estradiol were within normal ranges. All parameters of sperm health and serum markers returned to normal by 17 weeks.
The indenopyridine derivative RTI-4587-073(l) was studied in miniature stallions. 84 They noted severe oligoasthenozoospermia with high numbers of immature germ cells in the ejaculate. They also noted increased FSH concentrations in the treated stallions, reflecting the drugs activity on Sertoli cells. The semen parameters and serum gonadotropin concentrations were fully reversible in about 70 days.

Lonidamine derivatives
Analogues of Lonidamine, a chemotherapy drug, have been analyzed as potential reversible male birth control agents. Adjudin, one such Lonidamine analogue, is derived from 1H-indazole-3-carboxylic acid. It works by disrupting the Sertoli-germ cell junction by targeting the apical ectoplasmic specialization proteins. 85,86 This leads to the exfoliation of immature spermatids. Adjudin was studied in a rat model with two doses of 50 mg/kg weekly, which caused infertility in 100% of subjects. 87 Unfortunately, the target proteins of this drug were not specific to the gonads, and side effects included liver inflammation. This group attempted to increase testicular specificity by conjugating Adjudin with the carrier molecule FSH-β. They showed that this approach successfully increased gonadal selectivity and decreased the effective dose from 50 to 0.50 µg/kg in the rat model. 87 The BTB plays an important role in regulating all non-hormonal birth control agents. Recent rat studies have shown that overexpressing F5-peptide, an endogenous BTB modulator, can enhance the bioavailability of Adjudin in the testis and lower the needed dose to induce reversible infertility. 88 By lowering the effective dose of Adjudin, this may lower the systemic side effects to an acceptable level. To date, there are no published in vivo human studies with Adjudin.
Gamendazole is a Lonidamine analogue currently being studied for its antispermatogenic properties. This molecule works by inhibiting the heat shock protein HSP90AB1 and the eukaryotic translation elongation factor EEF1A1, leading to increased Interleukin 1α production by Sertoli cells. 89 Interleukin 1α disrupts the Sertoli cellgerm cell junction, leading to premature exfoliation of immature spermatids, similar to Adjudin. There is also a noted decrease in Inhibin B and associated increase in FSH. Initial rat studies demonstrated 100% infertility 3 weeks after a single dose of 6 mg/kg. 90 Unfortunately, fertility only returned to 57% of the initial cohort. Doses of 3 mg/kg resulted in 66% infertility and 100% return of fertility. Further studies into dosing and reversibility need to be completed prior to human studies.

Sperm ion channel blockers
CatSper is a calcium ion channel specific to sperm flagella and is essential for sperm motility. 91,92 It has roles in flagella hyperactivity, chemotaxis toward the female ovum, capacitation, and the acrosome reaction. 92 There are four CatSper channels that have all been shown to be critical to male fertility in mouse knock-out models. 91,93,94 Below are several promising sperm CatSper blocker targets. Other sperm-specific ion channels, such as the potassium channels KSper and SLO3, are also being investigated as potential targets.

RU1968
RU1968 is a synthesized inhibitor of CatSper and SLO3. 95 Cross-species studies, including sperm from humans, mice, and sea urchins, showed inhibition of CatSper with a 15-fold higher potency than SLO3. 95 No toxic side effects were seen in human sperm. The mechanism of action is still being elucidated, but RU1968 was shown to inhibit the pro-motility response caused by progesterone in the female reproductive tract. 95,96 HC-056456 HC-056456 is a CatSper blocker shown to reversibly and selectively decrease ion transit through the CatSper channel of patch-clamped sperm. 91 When treated with HC-056456, human sperm loses flagellar hyperactivity. 91 In vivo studies in a mouse model found decreased fertilization in HC-056456 treated sperm when inseminated into the uterus. 97 This was the first in vivo study to look at CatSper blockers as a potential male contraceptive in a mammalian model.

SLO3 channel inhibitors
SLO3 is a sperm-specific potassium channel that regulates calcium entry through the CatSper channel. 98 SLO3 works by hyperpolarizing the cell during capacitation to allow for calcium entry via CatSper. It may also work indirectly by altering the pH of sperm. Knock-out mice lacking SLO3 are infertile. 98 Research is being done to look at potential SLO3 inhibitors, such as quinine and quinidine, but to date there are no human trials. 99 Pristimerin and lupeol Pristimerin and lupeol are plant triterpenoids that are thought to inhibit sperm hyperactivation via the CatSper channel. Pristimerin is an isolate from Tripterygium Wilfordii, and Lupeol is isolated from dandelion root, aloe vera, and mangos. They are thought to work by binding CatSper and blocking activation by progesterone and pregnenolone. 100 Further research has cast doubt on the efficacy of these medications at inhibiting sperm hyperactivation, and further research is still needed to determine their potential as male contraceptives. 96

Small molecule inhibitors
An emerging area of interest is small molecule inhibitors of the spermatogenesis pathway. These molecules act by blocking the action of target proteins, typically enzymes. Below are examples of small molecule inhibitors that act in the male reproductive tract.

JQ1
JQ1 is a small molecule inhibitor of a testicular bromodomain protein named BRDT. 101 Bromodomain proteins are involved in epigenetics by assisting with histone acetylation, chromatin remodeling, and recruiting transcription factors. 102 BRDT knock-out mice are healthy but sterile. 103 Mouse models suggest that JQ1 leads to infertility by reducing the number of spermatozoa, sperm motility, and seminiferous tubule volume without altering serum hormone levels. 101 Male mice pretreated with JQ1 for 6 weeks were unable to procreate when caged with female mice continuously. 104 These effects were reversible, but JQ1 had off-target effects on non-testicular bromodomain proteins. 105 Future research will need to advance testicular specificity.

EP055
EP055 is a small molecule inhibitor of Eppin, an epididymal protease inhibitor. 106 Eppin is secreted by Sertoli cells and binds to the sperm surface in the testes. It is bound by semenogelin from the seminal vesicle during ejaculation to modulate sperm motility. Eppin then regulates PSA enzyme activity to hydrolyze semenogelin and induce progressive motility. 107 In male nonhuman primates immunized with Eppin, 78% developed high titers to Eppin and were infertile. After cessation of immunocontraception, 71% recovered fertility. 108 Further primate studies showed that infusion with low dose EP055 (75-80 mg/kg) followed by high dose (125-130 mg/kg) led to no normal sperm motility 30 h after initial infusion and full recovery of sperm motility by 18 days. 106

Calcineurin inhibitors
Cyclosporine A and FK506 (tacrolimus) are calcineurin inhibitors used as immunosuppressant drugs. 109 Their adverse effect on male fertility has been observed and led to research into their efficacy as reversible non-hormonal male contraceptives. The sperm-specific calcineurin subunits PPP3CC and PPP3R2 are potential targets for inhibition, as mice with knock-out in these genes are infertile with reduced sperm motility. 109,110 The mechanism is believed to be the inflexibility of the sperm midpiece. When healthy mice are treated with Cyclosporine A or FK506, they develop defects of sperm motility and morphology within 4-5 days. These effects are reversible 12 journals.sagepub.com/home/reh Therapeutic Advances in Reproductive Health 17 within 7 days of stopping the medication. 109 Testis-specific calcineurin inhibitors may be a viable form of reversible male contraception. Other molecules essential to the sperm calcineurin pathway may also be targetable. Inhibition of SPATA33, which localizes calcineurin to the mitochondria, has also led to infertile knock-out mice with reduced sperm motility. 111

Inhibitors of vasal peristalsis
Inhibition of the peristalsis of sperm through the male reproductive tract is an additional mechanism that has been explored for possible male contraceptives. Early studies explored phenoxybenzamine, an alpha-1-adrengergic antagonist, for its contraceptive characteristics through the selective blockade of the longitudinal muscles of the vas deferens and inhibition of peristalsis. 112 It was previously shown to cause reversible infertility of rats and block ejaculation in a small cohort of adult males. 113,114 Studies have explored other alpha-1-adrenergic antagonists as well, namely prazosin and tamsulosin. Despite prazosin's debated efficacy as a contraceptive agent, tamsulosin has previously been shown to decrease sperm concentration; albeit with side effects such as dizziness and orthostatic hypotension. [115][116][117][118][119] To date, there have been no large-scale clinical trials evaluating alpha-1-adrenergic antagonists as potential male contraceptives. Most recently, P2X1-purinoreceptor antagonists have been postulated as another option. P2X1-purinoreceptors are adenosine triphosphate (ATP) ligand-gated cation channels expressed on the cell membranes of smooth muscle cells along the vas deferens, and their absence leads to impaired peristalsis and reduced sperm concentration in the semen. 120,121 Eise et al. 122 showed that a selective blockage of P2X1-purinoreceptors, through an oral agent in rats, led to a reduced number of pregnancies during mating. It has yet to be further evaluated in large-scale clinical trials as well.

Triptonide
Triptonide is a traditional Chinese herb produced from the vine of T. Wilfordii, also known as thunder god vine. Observational studies looking at Tripterygium use for rheumatoid arthritis in humans showed lower sperm counts. 123 Triptonide use in mice and primates caused morphological defects and sperm with no forward motility. 124 Infertility was seen by 3-4 weeks in male mice and 5-6 weeks in cynomolgus monkeys after initiation. Both species regained fertility in 4-6 weeks after stopping the medication. There were no serious systemic effects or changes in serum markers in these studies. It appears to disrupt spermatogenesis by targeting the interaction between junction plakoglobin and SPEM1 (spermatid maturation 1) receptor on the sperm. 124 Mice with SPEM1 gene knock-out have deformed sperm with a characteristic kinked head wrapped around by the mid-section of the tail. 125 Naturally occurring compounds causing infertility may continue to offer insight into future male contraceptive pathways.

Gendarussa leaves
Justicia gendarussa Burm f. (Acanthaceae), colloquially known as the gendarussa leaf, is a medicinal plant local to tropical areas in southeast Asia and the Pacific islands and has been used as a method of male contraception by many from the region. 126 It has also been explored for its potential use in a variety of conditions, including, but not limited to, hypertension, AIDS, arthritis, and liver disease. [127][128][129][130] Specifically, the active ingredient, flavonoid gendarusin A, has shown to reversibly inhibit the activity of spermatozoa hyaluronidase, an enzyme crucial to sperm penetration of the cumulus oophorus during the acrosome reaction. [131][132][133] The chemical make-up of the leaf has been explored previously, but its potential use as a male contraceptive has yet to be demonstrated in dedicated preclinic contexts and clinical trials. [134][135][136] Histone demethylases Histone demethylation plays a crucial role in germ cell regulation in both mice and humans. 137 KDM5B, a histone demethylase, and several other small molecules that target histone demethylases (CPI-455, PBIT, and KDM5-C70) have been identified as potential targets for male contraception ( Table 2).

Identifying future non-hormonal targets
There are more than 1500 human genes associated with male infertility, of which 200 are believed to be reproductive tract specific. 138 These genes and proteins offer a host of enticing journals.sagepub.com/home/reh 13 Table 2. Summary of novel non-hormonal agents.   139 have recently reported on the development of a searchable database to help researchers identify high-quality contraceptive targets (Contraceptive and Infertility Target DataBase, https://www.citdbase.org). Current advances in PROTAC (proteolysis targeting chimera) and CRISPR (clustered regularly interspaced short palindromic repeats) technologies have unlocked the ability to eliminate specific proteins and knockout specific genes, respectively.

Category and drugs Citation
PROTACs eliminate proteins via the ubiquitinproteasome pathway and allow for the degradation of enzymatic and non-enzymatic proteins along the spermatogenesis pathway. CRISPR, on the contrary, allows us to study genes involved with the different stages of spermatogenesis. The ideal targets are specific to the testes and have no paralogs. 138 Table 3 shows a subset of promising targets for these technologies that have been separated into the stage of spermatogenesis where they function. 138 Table 3. Selection of most promising non-hormonal target genes.

Implantable sperm valve
The Bimek SLV is an implantable device that occludes the vas with a reversible valve mechanism. 140 The device is implanted in the outpatient setting, and the patient is able to operate the device with a palpable switch. It was first implanted in humans in 2009 and is currently undergoing further human clinical trials. This device theoretically allows patients to switch their fertility on and off but would require a wash out period each time the device is closed, similar to the waiting period required after vasectomy.

Polymer injectables
Another area of active research is reversible vasal occlusion with polymer injectables. RISUG ® (reversible inhibition of sperm under guidance) is an ultrasound guided, sterile styrene maleic anhydride polymer injected into the vas. RISUG is approved in India for permanent sterilization and has been shown to be reversible in animal models. 141 It is currently undergoing Phase III clinical trials. Vasalgel and Echo-V are two other polymer injectables currently in development. 142 These are promising future products for reversible, procedural male sterilization.

Conclusion
Reliable family planning is an essential element of modern society. For much of human history, the burden of unintended pregnancies have fallen disproportionately on the mother. Recent concerns regarding access to legal abortion has sparked a renewed interest for reliable male factor contraceptives beyond surgical sterilization and condoms. Modern efforts to develop a male birth control agent date back to the 1930s, and initially focused on altering the hypothalamic-pituitary-testes axis. Hormonal contraceptives face multiple barriers including systemic side effects, challenges with dosing regimens, route of delivery, and the public stigma of anabolic steroid abuse. Despite these challenges, some of the most widely studied and accepted potential male contraceptives are novel hormonal agents, such as DMAU and nestorone/ testosterone gels. The recognition of retinoic acid receptors as potential reversible male contraceptives in the 1950s opened the door to exploring various non-hormonal agents. These novel nonhormonal contraceptives can target spermatogenesis at any stage of development, but common targets include Sertoli cell-germ cell interactions, sperm ion channels, testicular epigenetics, and an assortment of small molecule inhibitors. Several non-hormonal agents have entered human trials. The identification of reproductive tract-specific genes associated with male infertility, coupled with advances in PROTACs and CRISPR, may lead to more targeted drug development in the future. Despite multiple promising contenders over the last half century, no novel birth control agents have garnered regulatory approval in the United States or abroad. Future research is needed to achieve novel forms of male birth control.

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